CODEWITHSUNDEEP
c#asynchronous
mmokrzycki
mmokrzycki

Reputation: 82

Download files asynchronously, after that resync

I tried to download asynchronously certain files, using tips from MSDN and from here, but my attempts usually end with deadlock or errors caused by aplication not waiting for the end of download. Below I pasted sample code that doesn't work but hopefully explains my intentions. I will be grateful for your help. 

public void SomeMethod() // which must be called synchronously
    {
        // Determine which files to download
    List<FileRequest> fileRequests = Determine();

    var test = DownloadFilesAsync(fileRequests);
    test.Wait();

    // After that do something else with downloaded files synchronously
}

public async Task DownloadFilesAsync(List<FileRequest> fileRequests)
    {
        await Task.WhenAll(fileRequests.Select(fileRequest =>
DownloadFileAsync(fileRequest))).ConfigureAwait(false);
    }

public async Task DownloadFileAsync(FileRequest fileRequest)
    {
        using (WebClient client = new WebClient())
        {
            await client.DownloadFileTaskAsync(fileRequest.url,fileRequest.downloadPath).ConfigureAwait(false);
        }
    }

Upvotes: 0

Views: 467

Answers (1)

Nkosi
Nkosi

Reputation: 246998

Using the test.Wait(); is blocking the async method

Best practice in using async is to use await all the way through the method. Don’t mix blocking and async code.

public async Task SomeMethod() {
    // Determine which files to download
    List<FileRequest> fileRequests = Determine();

    //this will allow the down load to not lock the ui
    await DownloadFilesAsync(fileRequests);

    // After that do something else with downloaded files synchronously
    //...
}

public async Task DownloadFilesAsync(List<FileRequest> fileRequests) {
    await Task.WhenAll(fileRequests.Select(fileRequest =>
        DownloadFileAsync(fileRequest))).ConfigureAwait(false);
}

public async Task DownloadFileAsync(FileRequest fileRequest) {
    using (WebClient client = new WebClient()) {
        await client.DownloadFileTaskAsync(fileRequest.url,fileRequest.downloadPath).ConfigureAwait(false);
    }
}

Source - Async/Await - Best Practices in Asynchronous Programming

Upvotes: 1

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